Optical coherence tomography (OCT) is the most recent imaging innovation in ophthalmology used to study the structure of the eye. Even more recent applications of this type of scan have been to study the anterior portion of the eye, but the primary usage has been for the evaluation of the retina, and more specifically the back of the eye. This portion of the eye is called the posterior pole and includes the macula and the optic nerve.
There is a great similarity between ultra-sonography and optical coherence tomography, in that they both image by reflecting an impulse of energy onto the subject matter being studied and analyzing the energy reflected back. The difference is that sonography uses sound waves, which can penetrate opaque matter, and OCT uses light waves, which only penetrate translucent matter. Because light waves have a much shorter wave length than sound waves, there is much greater/better resolution in image presentation.
When to Use Optical Coherence Tomography
OCT is currently most often used to examine the macula in ARMD (age-related macular degeneration). In this condition abnormalities in the retinal tissue can be identified and measured. These abnormalities include breaks in the outer retina tissues, accumulation of metabolic by-products, fluid under or in the retina, scar tissue under the retina or on the surface of the retina, and traction on the macula and/or surrounding retina. Because of its high resolution the individuals layers of the retina can be identified, aiding in the diagnosis of the macular and retinal condition.
When used to evaluate the optic nerve, the OCT quantitates the amount of optic nerve damage, nerve swelling, and the degree of nerve fiber loss and location. It can also reveal vitreous traction on the optic nerve.
An OCT is Non-Invasive and Does Not Hurt
The examination using the optical coherence tomographer is entirely benign, without any risk factors. The patient is positioned in front of the OCT instrument and the head is placed in a frame for stabilization. The instrument is then placed in front of the study eye, which is not touched, and the patient is asked to look at a light target. The Instrument, using a low intensity laser beam, scans the eye within seconds, and data is then printed out, including a cross-sectional view of the retina, a topographic view and the retinal thickness.
How often is an OCT Needed?
Besides aiding in the diagnosis of retinal and macular disease, the optical coherence tomographer is invaluable in evaluating the progression of a disease, as well as, showing if the condition is responding to treatment. In some conditions, it may be used at each visit as continuation of treatment is based on the results of the study. This is particularly true in the case of treating Age-related macular degeneration when an endpoint is determined when there is no longer any evidence of new vessel and all fluid has been reabsorbed. The OCT has significantly decreased the need for fluorescein angiography, where an IV injection of fluorescein is required, followed by numerous photographs. This procedure takes much more time and is more invasive. It also has an additional risk, in that the patient may have an adverse effect to the fluorescein.
The use of the OCT has become a much more utilized imaging device in ophthalmology, but there continues to be a need for fluorescein angiography in certain cases where the cause of the macular damage is not revealed by the OCT.